1. Field of the Invention
The present invention relates to an exposure apparatus and a device fabrication method.
2. Description of the Related Art
In recent years, as techniques of fabricating semiconductor devices, microfabrication techniques have made remarkable progress. In the optical processing technology (photolithography), an exposure apparatus having a resolution on the submicron order has become the mainstream. To further improve the resolution, the numerical aperture (NA) of a projection optical system is increased and the wavelength of exposure light is shortened. As the wavelength of exposure light shortens, a light source used in an exposure apparatus shifts from a high-pressure mercury lamp which uses the g-line or i-line to, for example, a KrF excimer laser or an ArF excimer laser. However, as the wavelength of exposure light shortens, the type of usable glass material is limited, so it becomes more difficult to correct the chromatic aberration of a projection optical system for the wavelength of alignment light. Hence, the recent exposure apparatus often employs an off-axis alignment optical system (OA optical system) that is not adversely affected by the chromatic aberration of a projection optical system.
In a projection optical system and OA optical system, a change in temperature is known to adversely affect the imaging performance and alignment performance (that is, degrade the imaging performance and alignment performance). Hence, to suppress degradation in alignment performance due to a change in temperature in the vicinity of a sensor in the OA optical system, Japanese Patent Laid-Open No. 10-135117 proposes a technique of enclosing the sensor using an enclosing member having a predetermined airtightness to control the temperature in this enclosing member. In the technique described in Japanese Patent Laid-Open No. 10-135117, during exposure, heat propagated from, for example, a projection optical system, substrate, or reticle (mask) is shielded using the enclosing member, and the temperature in this enclosing member is controlled, thereby maintaining the temperature in the vicinity of the sensor constant.
Also, the OA optical system generally uses, for example, a halogen lamp (wide wavelength band light source) as an alignment light source, and selects a wavelength band used for measurement from a plurality of wavelength bands in accordance with, for example, the measurement conditions. The halogen lamp is known to have a positive correlation between the ON voltage and the life: the life is shorter when the ON voltage is relatively high than when the ON voltage is relatively low. Therefore, in the OA optical system, to achieve both a long life and energy saving, the halogen lamp is turned on at a relatively high ON voltage to ensure a required illuminance in the measurement period, and is turned on at a relatively low ON voltage in the non-measurement period (that is, the amount of light is set relatively small in the non-measurement period). Also, in the OA optical system, alignment light from the alignment light source is guided to an optical system (sensor) of the OA optical system via a fiber. This is to separate the optical system of the OA optical system and the alignment light source as a heat source to prevent expansion/contraction of the optical system of the OA optical system due to a change in temperature, thereby suppressing degradation in alignment performance.
However, since the alignment light guided via the fiber contains energy as well, the optical system of the OA optical system expands/contracts upon irradiation with the alignment light, thus hindering the OA optical system from ensuring high alignment performance required these days. Especially in the OA optical system, the ON voltage of the halogen lamp is changed between the measurement period (during measurement) and the non-measurement period (during non-measurement), the amount of alignment light (amount of heat) which enters the OA optical system is different between the measurement period and the non-measurement period. As a result, the change in temperature of the OA optical system in the measurement period is considerable, and this adversely affects the alignment performance of the OA optical system.
Although it is also possible to apply the technique described in Japanese Patent Laid-Open No. 10-135117 so that the optical system of the OA optical system is enclosed to control the temperature in this optical system, a temperature control mechanism, for example, is additionally required, leading to increases in size and cost of the configuration of the OA optical system.